1. Field of the Invention
This invention relates to coil presses for setting the wrapped insulation of a coil for a large electric machine such as a generator or motor. Such a coil (referred to hereinafter as "a coil of the type described") comprises a loop of copper or other good conductor having parallel straight legs which fit into slots in the rotor or stator or like part of the electric machine. The cross section of the material of the loop is usually rectangular. A typical loop may have its straight legs there or four feet long and separated by two or three feet, but of course the size of the loop depends on the size of the electric machine, which can be anything from a motor of one or a few horsepower to a generator for a power station. A typical cross section of the coil material is one inch by a quarter inch (which may be solid or assembled from a number of wires) although this too varies according to the size, type and rating of the electric machine.
2. Description of the Prior Art
The conductive material of the coil has of course to be electrically insulated prior to assembly in the electric machine. This has in the past been done by wrapping it with mica strip, but more recently thermoplastic or thermosetting resin-impregnated tape has been used. This tape is wound helically around the coil, at least over the straight legs thereof which fit into the rotor or stator slots. At least the wrapped tape on the straight legs is then subjected to heat and pressure, which plasticises and consolidates the resin so as to impart a hard smooth surface thereto and to bond the layers of tape together and to delimit the cross-sectional dimensions thereof so as to conform to the dimensions of the slot of the electric machine.
Although the characteristics of the new resin-impregnated tapes are quite different from those of the old mica type, no special machinery has been available for setting them on the coils, and the same methods as were used for the mica strips are still in use. It has been found, however, that these old methods and old machinery have certain disadvantages when used with the new tapes. In the old methods and old apparatus, for example, the coil was put one leg at a time into a press. It has been found that, with the new resin-impregnated tapes, small but important distortions can occur as a result of the greater tendency of the resin to flow under heat and pressure, the precise nature of which distortions depends upon the way the rest of the coil is supported by the part of it held in the press. This can lead to inaccuracies in the way the coil fits into the rotor or stator slot, which in turn can lead to undesirable machanical stresses otherwise known as bending stresses in the coils or inefficient performance of the machine.